1. Field of the Invention
This invention relates to a method of driving an actuator for displacing a mechanism connected thereto to a target displacement (position) and holding it there against a load, and more particularly to a method of driving such an actuator which controls the actuator output so as to utilize internal frictional force for holding the target position, thereby reducing the actuator load and energy consumption.
2. Description of the Prior Art
In the electromechanical field, various types of actuators are used for converting energy into movement of a load. In the control of this conversion, the friction in the actuator and the mechanism and the motion transmission system thereby has up to now been treated as a negative factor adversely affecting positioning accuracy and the like. This thinking is reflected, for example, in the frictional force compensation method described in Japanese Patent Publication No. 2(1990)-39,805.
When the mechanism with friction is positionally controlled and then held at a given position, the force balance established can be expressed by the equation below. (In this specification, the term "friction" is used to mean the phenomenon of kinematic energy being converted to heat by relative movement between mechanisms or members in a continuous system and the term "internal frictional force" is used to mean the force acting opposite to the direction of motion on the contacting surface of a mechanism or member moving relative to another mechanism or member. And the term "Displacement" is used to mean angle or distance. EQU Actuator output=External load+Internal frictional force (Eq.1)
Assuming that the internal frictional force can assume an arbitrary value between a negative value F1 and a positive value F2, i.e. F1&lt; Internal frictional force&lt; F2, it holds from the equation that EQU F1&lt;Actuator output-External load&lt;F2 (Eq.2)
which can be rewritten as EQU F1+External load&lt;Actuator output&lt;F2+External load (Eq.3)
In other words, the forces are balanced and the displacement is held insofar as the actuator output is within the range defined by Equation 3.
As shown in FIG. 21, in PD and other conventional types of positional control, the value of the actuator output is totally ignored insofar as the displacement is held by maintaining the force balance indicated by Equation 3. Therefore, assuming the amount of energy consumed by the actuator to be proportional to the square of the actuator output, the amount of energy consumed by the actuator during hold may, in the worst case, become the larger of the square of (external load+F1) and the square of (external load+F2). Although method of saving energy by discontinuing control has ben used and thus reducing the actuator output to zero during hold, it is clear from Equation 3 that this expedient can be used only when EQU 9F1+External load&lt;0,
and EQU F2+External load&gt;0 (Eq.4)
Reduction of actuator energy consumption is particularly important in autonomous mobile robots and the like.